'\"
'\" Generated from file 'aycock\&.man' by tcllib/doctools with format 'nroff'
'\" Copyright (c) 2006 by Kevin B\&. Kenny <kennykb@acm\&.org>
'\" Redistribution permitted under the terms of the Open Publication License <http://www\&.opencontent\&.org/openpub/>
'\"
.TH "grammar::aycock" n 1\&.0 tcllib "Aycock-Horspool-Earley parser generator for Tcl"
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.SH NAME
grammar::aycock \- Aycock-Horspool-Earley parser generator for Tcl
.SH SYNOPSIS
package require \fBTcl  8\&.5\fR
.sp
package require \fBgrammar::aycock  ?1\&.0?\fR
.sp
\fB::aycock::parser\fR \fIgrammar\fR ?\fB-verbose\fR?
.sp
\fIparserName\fR \fBparse\fR \fIsymList\fR \fIvalList\fR ?\fIclientData\fR?
.sp
\fIparserName\fR \fBdestroy\fR
.sp
\fIparserName\fR \fBterminals\fR
.sp
\fIparserName\fR \fBnonterminals\fR
.sp
\fIparserName\fR \fBsave\fR
.sp
.BE
.SH DESCRIPTION
.PP
The \fBgrammar::aycock\fR package
implements a parser generator for the class of parsers described
in John Aycock and R\&. Nigel Horspool\&. Practical Earley Parsing\&.
\fIThe Computer Journal,\fR \fI45\fR(6):620-630, 2002\&.
\fIhttp://citeseerx\&.ist\&.psu\&.edu/viewdoc/summary?doi=10\&.1\&.1\&.12\&.4254\fR
.SH PROCEDURES
The \fBgrammar::aycock\fR package exports the single procedure:
.TP
\fB::aycock::parser\fR \fIgrammar\fR ?\fB-verbose\fR?
Generates a parser for the given \fIgrammar\fR, and returns its
name\&.  If the optional \fB-verbose\fR flag is given, dumps verbose
information relating to the generated parser to the standard output\&.
The returned parser is an object that accepts commands as shown in
\fBOBJECT COMMAND\fR below\&.
.PP
.SH "OBJECT COMMAND"
.TP
\fIparserName\fR \fBparse\fR \fIsymList\fR \fIvalList\fR ?\fIclientData\fR?
Invokes a parser returned from \fB::aycock::parser\fR\&. \fIsymList\fR is
a list of grammar symbols representing the terminals in an input
string, and \fIvalList\fR is a list of their semantic values\&. The
result is the semantic value of the entire string when parsed\&.
.TP
\fIparserName\fR \fBdestroy\fR
Destroys a parser constructed by \fB::aycock::parser\fR\&.
.TP
\fIparserName\fR \fBterminals\fR
Returns a list of terminal symbols that may be presented in the
\fIsymList\fR argument to the \fBparse\fR object command\&.
.TP
\fIparserName\fR \fBnonterminals\fR
Returns a list of nonterminal symbols that were defined in the
parser's grammar\&.
.TP
\fIparserName\fR \fBsave\fR
Returns a Tcl script that will reconstruct the parser without
needing all the mechanism of the parser generator at run time\&.
The reconstructed parser depends on a set of commands in the
package \fBgrammar::aycock::runtime\fR,
which is also automatically loaded
when the \fBgrammar::aycock\fR package is loaded\&.
.PP
.SH DESCRIPTION
The \fBgrammar::aycock::parser\fR command accepts a grammar expressed as
a Tcl list\&.  The list must be structured as the concatenation of a set
of \fIrule\fRs\&. Each \fIrule\fR comprises a variable number of
elements in the list:
.IP \(bu
The name of the nonterminal symbol that the rule reduces\&.
.IP \(bu
The literal string, \fB::=\fR
.IP \(bu
Zero or more names of terminal or nonterminal symbols that
comprise the right-hand-side of the rule\&.
.IP \(bu
Finally, a Tcl script to execute when the rule is reduced\&.
Within the given script, a variable called \fB_\fR contains a list of
the semantic values of the symbols on the right-hand side\&. The value
returned by the script is expected to be the semantic value of the
left-hand side\&.  If the \fIclientData\fR parameter was passed to the
\fBparse\fR method, it is available in a variable called
\fBclientData\fR\&.  It is permissible for the script to be the empty
string\&.  In this case, the semantic value of the rule will be the same
as the semantic value of the first symbol on the right-hand side\&.  If
the right-hand side is also empty, the semantic value will be the
empty string\&.
.PP
Parsing is done with an Earley parser, which is not terribly efficient
in speed or memory consumption, but which deals effectively with
ambiguous grammars\&.  For this reason, the \fBgrammar::aycock\fR package is
perhaps best adapted to natural-language processing or the parsing of
extraordinarily complex languages in which ambiguity can be tolerated\&.
.SH EXAMPLE
The following code demonstrates a trivial desk calculator, admitting
only \fB+\fR, \fB*\fR and parentheses as its operators\&.  It also
shows the format in which the lexical analyzer is expected to present
terminal symbols to the parser\&.
.CS


set p [aycock::parser {
    start ::= E {}
    E ::= E + T {expr {[lindex $_ 0] + [lindex $_ 2]}}
    E ::= T {}
    T ::= T * F {expr {[lindex $_ 0] * [lindex $_ 2]}}
    T ::= F {}
    F ::= NUMBER {}
    F ::= ( E ) {lindex $_ 1}
}]
puts [$p parse {(  NUMBER +  NUMBER )  *  ( NUMBER +  NUMBER ) }  {{} 2      {} 3      {} {} {} 7     {} 1      {}}]
$p destroy

.CE
The example, when run, prints \fB40\fR\&.
.SH KEYWORDS
Aycock, Earley, Horspool, parser, compiler
.SH KEYWORDS
ambiguous, aycock, earley, grammar, horspool, parser, parsing, transducer
.SH CATEGORY
Grammars and finite automata
.SH COPYRIGHT
.nf
Copyright (c) 2006 by Kevin B\&. Kenny <kennykb@acm\&.org>
Redistribution permitted under the terms of the Open Publication License <http://www\&.opencontent\&.org/openpub/>

.fi
